1. Field of the Invention
The present invention relates to a modified-flame thermionic detector for gas chromatographs, of the type including, in a way known in itself, a burner nozzle fed with eluent gas from the chromatograph and with a combustible gas, a collecting electrode coaxial with the flame and connected to a recorder of the current emitted by the same, as well as an excitation electrode comprising an alkali source of thermions, placed on the flame axis, between the latter and the collector.
2. Description of the Prior Art
Modified-flame thermionic detectors of the above mentioned type are already known and used in gas chromatography, for instance as indicated in the U.S. Pat. No. 3,852,037. The detector illustrated in this patent includes an excitation electrode consisting of a glass containing alkali, which is heated to obtain ion emission. In this U.S. patent there is the possibility of modifying the ion-emitting alkali substance, of modifying the nozzle connection to the negative potential of an electric circuit in continuous current or to ground respectively, as well as especially of modifying the combustible gas flow (and also the combustion gas, for instance air) supplied to the nozzle, to determine higher or lower selectivity of the detector with respect to certain substances, in particular to determine the possibility of commutating the detector from its functioning as normal flame ionization detector (FID) to a detector with particular sensibility to halogens and phosphorus. There is also indicated the possibility of using the detector as a detector sensitive to nitrogen compounds, though without use of the flame but with heating by means of electric resistance of the excitation electrode. This detector according to U.S. Pat. No. 3,852,037 allows obtaining chromatograms in which the presence of a certain component is found out, in particular phosphorus and/or nitrogen compounds, but however, this is achieved by considerably modifying the operating conditions of the detector which requires a deep study of the situation from the operator, and in practice at least rough pre-existing knowledge of the sample to be analysed on the basis of which the detector must be adjusted.
In case the sample is unknown, it is necessary to perform various subsequent test analyses until one finds the best conditions of electrical connection of the detector parts, of supply of gases to the nozzle and of flame ignition or not, which involves heavy losses of time and the possibility of obtaining wrong results.
Another detector of the known technique, which was used to try to enhance the sensitivity to certain compounds, is illustrated by the U.S. Pat. No. 3,585,003, which describes a detector having a nozzle and a collecting electrode, as well as a couple of so-called "barrier" electrodes, disposed on one side and on the other of the collector. By varying the potential of the barrier electrodes, in this case under constant conditions of gas supply to the flame nozzle, it is possible to obtain a certain variation in sensitivity, particularly to phosphorus-containing compounds. This variation is, however, extremely reduced and of little significance, as it can be noticed in the chromatograms reported in the text of the mentioned U.S. patent.
Finally, another type of detector is known, illustrated in U.S. Pat. No. 3,677,709, which includes a flame nozzle and a ring-shaped collecting electrode, which supports a piece of alkali salt acting as ion-emitting substance. According to this patent, the flame nozzle and the collecting electrode are reciprocally adjustable, so that it is possible, by a suitable reciprocal movement, to change the salt supported by the collecting electrode and to modify the flow in particular of the combustible gas (usually hydrogen), to obtain a certain sensitivity particularly to carbon-containing compounds, which may also give negative peaks on the chromatogram (see FIGS. 6 and 7 of the U.S. Pat. No. 3,677,709).
From the preceding review of the present techniques in the field, it becomes clear that, even if several attempts have been carried out to obtain flame detectors having different sensitivities and variable sensitivity with respect to compounds containing different elements, the results obtained have been rather poor, in that it was generally possible to enhance sensitivity to compounds containing only one particular element, and this with extremely complex, difficult and uncertain regulations.